The present invention relates to a charge pump circuit for stepping up or down an input voltage to output a stepped-up or stepped-down output voltage, and more particularly, to a technique of preventing overcurrent-caused circuit breakdown.
Conventionally, charge pump circuits are widely known as power supply circuits that can be implemented in a comparatively simple configuration. The charge pump circuits include step-up (boost) charge pump circuits that step up an input voltage to output a positive output voltage and step-down charge pump circuits that step down an input voltage to output a negative output voltage.
FIG. 10 shows a configuration of a general step-up charge pump circuit, which executes a charge storing operation for storing charge in a pumping capacitance C81 and a charge transfer operation for transferring the charge stored in the pumping capacitance C81 alternately, to thereby output a positive output voltage Vout (=2Vdd) having a voltage value twice as large as the power supply voltage Vdd.
During a charge storing period (during which the charge storing operation is executed), drive circuits 801, 802, 803 and 804 respectively output the ground voltage Vss, the power supply voltage Vdd, the power supply voltage Vdd and the output voltage Vout. With these voltages, drive transistors T81 and T82 are ON while drive transistors T83 and T84 are OFF, resultantly allowing the pumping capacitance C81 to store an amount of charge (positive charge) corresponding to the voltage difference between the power supply voltage Vdd and the ground voltage Vss.
During a charge transfer period (during which the charge transfer operation is executed), the drive circuits 801, 802, 803 and 804 respectively output the output voltage Vout, the ground voltage Vss, the ground voltage Vss and the power supply voltage Vdd. With these voltages, the drive transistors T83 and T84 are ON while the drive transistors T81 and T82 are OFF, resultantly allowing the charge (positive charge) stored in the pumping capacitance C81 to be transferred to an output node Nout.
In the step-up charge pump circuit, the drive transistors T81 and T84 can be turned OFF by supplying the output voltage Vout (2Vdd) to the gates thereof. However, if a short-to-ground fault of the output node Nout (short-circuiting between the output node Nout and an unintentional low-voltage node (ground node, for example)) occurs, the output voltage Vout at the output node Nout may become lower than the power supply voltage Vdd. In such an event, the drive transistors T81 and T84 will not be turned OFF but be ON even if the output voltage Vout is supplied to the gates of the drive transistors T81 and T84. Hence, both the drive transistors T81 and T84 will be ON simultaneously, causing an overcurrent between an input node Nin and the output node Nout.
FIG. 11 shows a configuration of a general step-down charge pump circuit, which executes a charge storing operation and a charge transfer operation alternately, to thereby output a negative output voltage Vout (=−Vdd) stepped down from the ground voltage Vss by the value of the power supply voltage Vdd.
During the charge storing period, drive circuits 901, 902, 903 and 904 respectively output the power supply voltage Vdd, the ground voltage Vss, the ground voltage Vss and the output voltage Vout. With these voltage, drive transistors T91 and T92 are ON while drive transistors T93 and T94 are OFF, resultantly allowing a pumping capacitance C91 to store an amount of charge (negative charge) corresponding to the voltage difference between the power supply voltage Vdd and the ground voltage Vss.
During the charge transfer period, the drive circuits 901, 902, 903 and 904 respectively output the output voltage Vout, the power supply voltage Vdd, the power supply voltage Vdd and the ground voltage Vss. With these voltages, the drive transistors T93 and T94 are ON while the drive transistors T91 and T92 are OFF, resultantly allowing the charge (negative charge) stored in the pumping capacitance C91 to be transferred to an output node Nout.
In the step-down charge pump circuit, the drive transistors T91 and T94 can be turned OFF by supplying the output voltage Vout (−Vdd) to the gates thereof. However, if a short-to-power fault of the output node Nout (short-circuiting between the output node Nout and an unintentional high-voltage node (power supply node, for example)) occurs, the output voltage Vout at the output node Nout may become higher than the ground voltage Vss. In such an event, the drive transistors T91 and T94 will not be turned OFF but be ON even if the output voltage Vout is supplied to the gates of the drive transistors T91 and T94. Hence, both the drive transistors T91 and T94 are ON simultaneously, causing an overcurrent between an input node Nin and the output node Nout.
With occurrence of an overcurrent due to an unintentional voltage change at the output node Nout as described above, the charge pump circuit may possibly be broken down.
To address the above problem, Japanese Laid-Open Patent Publication No. 2004-320862 (Patent Document 1) discloses a DC-DC converter in which a constant voltage circuit capable of adjusting the output current amount is connected to an input node of a step-up charge pump circuit to restrict the current amount supplied to the input node, to thereby suppress an overcurrent from occurring with a short-to-ground fault. Specifically, the constant voltage circuit includes a voltage control transistor placed between an input terminal for receiving an input voltage and an output node connected to the input node of the charge pump circuit. The output current amount of the constant voltage circuit is adjusted by controlling the gate voltage of the voltage control transistor.
However, Patent Document 1 described above has the following problems. With a voltage drop occurring in the voltage control transistor of the constant voltage circuit, a voltage lower than the input voltage is supplied to the charge pump circuit. This degrades the step-up efficiency of the charge pump circuit.
Also, in Patent Document 1, it is necessary for the constant voltage circuit to have a current drive capability higher than the charge pump circuit. For example, to secure 500 mA as the output current of the charge pump circuit, the constant voltage circuit must output a current of 1 A or more. With this requirement of having such a high current drive capability, it is difficult to reduce the circuit scale of the constant voltage circuit.